1. Field of the Invention
The present invention relates to a sensor system for an in-line pipe inspection tool. Such an in-line pipe inspection tool is also known as a pipeline pig.
2. Summary of the Prior Art
It is known to inspect a pipeline from the inside using a pipeline pig which passes down the pipe. For magnetic inspection, the pig has permanent magnets defining pole pieces, which are positioned adjacent to the inner wall of the pipe. Those magnets then generate magnetic fields which magnetise the wall of the pipe. Sensors are provided between the magnetic poles, which detect the magnetic flux density at the internal surface of the pipe. The magnetic field in the pipe wall is normally constant, but is disturbed by a flux changing feature, such as a defect, weld bead or wall thickness change, and magnetic flux then leaks out of the pipe at such a feature, to be detected by the sensors. As the pipeline pig is driven along the pipe, the location of the pole pieces, and the sensors, moves along the pipe enabling the internal surface of the pipe to be inspected.
In known arrangements, the sensors may be mounted directly to the body of the pig, but it is more usual to mount them on a sensor carrier, which may also carry the pole pieces of the magnetic pole, and which sensor carrier is connected to the body of the pig by a deformable linkage. Such a deformable linkage permits the sensor carrier to move radially, to allow it to pass e.g. a deformation in the pipe. Thus, the sensor carrier usually forms a body conforming to part of the circumference of a cylinder, and the sensors are mounted on the carrier so as to extend around the circumference of the part-cylinder. Thus, as the pipeline pig moves along the pipeline, the sensors sense an arc of the pipeline, at a given position along the pipeline. Normally, a plurality of such sensor carriers, each with a plurality of sensors, are provided on the pipeline pig, so that the whole circumference of the pipeline can be sensed.